The present invention relates to a capacitive pressure sensor having a diaphragm structure and designed to capacitively detect a change in measurement pressure, and a method of manufacturing the same.
FIG. 8 shows the arrangement of a capacitor structure of a capacitive pressure sensor of this type disclosed in, e.g., Japanese Patent Laid-Open No. 63-305229. Referring to FIG. 8, a capacitor structure for a pressure converter is constituted by a support base plate 53 formed by bonding a glass wafer 52 to the upper surface of a silicon wafer 51, a first capacitor plate 54 and second capacitor plates 55a and 55b formed on the upper surface of the support base plate 53, a diaphragm 57 having hollow portions 56a and 56b formed in its lower and upper surfaces and arranged on the support base plate 53, a silicon wafer 59 having a glass wafer 58 bonded to its lower surface and arranged on the diaphragm 57, and a pressure introducing through hole 60 extending through the glass wafer 58 and the silicon wafer 59.
Since the conventional capacitive pressure sensor is constituted by a multi-layer bonding structure in which a plurality of layers, each consisting of silicon and glass wafers bonded to each other, are stacked on each other, the number of constituent components is large, and the diaphragm thickness and the gap can be reduced at most to about several .mu.m in practice. Also, reduction of the sensor in size results in reducing the capacitance thereof as well as degrading the sensitivity to a pressure.
As another type of sensor, a so-called thin-film diaphragm type pressure sensor is disclosed in Japanese Patent Laid-Open No. 63-298130. As shown in a cross-sectional view of FIG. 9, a diaphragm structure of this type of pressure sensor is obtained by forming an upper movable electrode 64 and a metal diaphragm 65 on a photosensitive glass substrate 62, on which a lower stationary electrode 61 is formed, through a hollow portion 63, and forming a pressure introducing small hole 66 in the lower surface of the photosensitive substrate 62 to communicate with the hollow portion 63. According to such a structure, the thickness of the diaphragm 65 and the gap size of the hollow portion 63 can be easily reduced to a submicron order. Therefore, this structure is very advantageous to reduce the sensor size.
A pressure sensor is generally required to have high resistance to an excessive pressure in addition to a function of converting a pressure into an electrical signal. For example, a general pressure sensor is required to have a protective function against both positive and negative excessive pressures. For this reason, in the case of the conventional pressure sensor described above, its structure provides a protective function against an excessive pressure applied in a direction to reduce the gap. However, since no stopper function is provided for a pressure applied in a direction to increase the gap, a separate stopper mechanism is required. In the above-described pressure sensor, however, as the size of the sensor is reduced, the displaceable span of the diaphragm becomes very narrow. Therefore, the dimensional tolerance required to mount the stopper mechanism is very limited. Such a small dimensional tolerance makes it very difficult to realize a compact pressure sensor.